Field of the Invention
The invention relates to a fuel cell system and a method for controlling the same. Particularly, the invention relates to a fuel cell system capable of stably outputting electric power and a method for controlling the same.
Description of Related Art
Development and application of energy have always been indispensable conditions of human life, however, the development and application of energy may cause increasing damage to the environment. Energy generated through a fuel cell technique has advantages of high efficiency, low noise and pollution-free, etc. which is an energy technology in line with a trend of the times. Types of the fuel cells are diversified, and the commonly used fuel cells are direct methanol fuel cells (DMFC) and proton exchange membrane fuel cells (PEMFC). However, both of the DMFC and the PEMFC generate power through fuel reaction, and fuel quantity and a speed of the fuel reaction may influence performance and stability of the whole fuel cell.
Taking the PEMFC as an example, the PEMFC is composed of a proton exchange membrane, an anode, and a cathode. The anode takes hydrogen as a fuel, and the hydrogen is transported to a flow channel of the anode through a pump and a flux control valve. When the hydrogen is reacted with a catalyst in the anode, heat is produced. The fuel of the anode is reacted with the catalyst to produce hydrogen ions and electrons, and a chemical formula thereof is:2H2→4H++4e−
The electrons generated during the anode reaction move to the cathode through a circuit, and the hydrogen ions move to the cathode by penetrating through the proton exchange membrane, and are further reacted with the electrons and oxygen to generate water, and a chemical formula thereof is:4H++4e−+O2→2H2O
The water generated at the cathode is recycled through a condensation system. On the other hand, since a flow rate of the hydrogen supplied to the anode is required to be stable during the reaction, a complicated valve and a pump are generally required to implement a flux control of the hydrogen.
The hydrogen serving as the fuel could be produced by a general hydrogen producing method and a chemical hydrogen storage technology is widely used in portable electronic devices. The chemical hydrogen storage technology generally produces hydrogen through reaction of a solid-state reactant and a liquid reactant. The solid-state reactant could be metal hydride or metal, for example, NaBH4, MgH2, CaH2 or Al. The liquid reactant includes liquid water, malic acid, citric acid, H2SO4, NaHCO3, CaCO3, etc. For example, water could be added to sodium borohydride for chemical reaction, so as to produce hydrogen. Then, the produced hydrogen is introduced to the fuel cell to react to generate electric power. Regarding the reaction of the sodium borohydride and water, the water could be added to the sodium borohydride or the sodium borohydride could be added to the water for reaction, and a chemical formula thereof is:NaBH4+2H2O→NaBO2+4H2 
According to the above chemical formula, it is known that the hydrogen could be continually produced in one chemical reaction until the chemical reaction between the solid-state reactant and the liquid reactant is completed. Since the hydrogen generated through the chemical reaction is unstable in flux, a flux control valve is generally used to stably output the hydrogen.
Moreover, when the water is added to the solid-state reactant, a large amount of the hydrogen is released. When the fuel cell receives a large amount of the hydrogen in a short time, an output voltage thereof is increased and a temperature thereof is sharply increased, which causes difficulty in the system control.
Presently, a valve is used to control the output of the hydrogen in order to achieve a precise control, though the valve is quite expansive. Moreover, if the output of the hydrogen is controlled without the valve, following problems are encountered:                1. It is unable to be operated at different ambient temperatures. In case of a low ambient temperature, a large amount of chemical reactions is required to produce hydrogen and maintain the temperature, and in case of a high ambient temperature, logic for controlling the flux of the hydrogen is different due to better reactivity. On the other hand, if the temperature is excessively high, the system is hard to decrease the temperature to an optimal temperature for the fuel cell.        2. The flux of the hydrogen cannot be controlled. A large amount of hydrogen may cause sharp increase of the voltage and temperature, and a small amount of hydrogen results in a low voltage. If there is no good controlling method for the flux of the hydrogen, a service life of the fuel cell is quickly decreased.        3. According to a general control mode, the temperature range of the fuel cell is enlarged, which may cause an excessively high or low temperature. Since the temperature directly influences the performance of the fuel cell in outputting the electric power, a fuel usage rate of the fuel cell is decreased.        
Moreover, U.S. Patent Publication No. 20090214904 discloses an integrated fuel and a fuel cell, in which a rate for introducing water or vapor to a solid fuel is controlled by controlling an aperture size and using a film material. Moreover, Taiwan Publication No. 201004018 discloses a fuel supplying method of a fuel cell. In addition, U.S. Patent No. 20070196700 discloses a controlling method for fuel supply.